Chilling effect on termination of reproductive diapause in Listronotus maculicollis (Coleoptera: Curculionidae)

Gradually Increasing the Temperature Reduces the Diapause Termination Time of Trichogramma dendrolimi While Increasing Parasitoid Performance

Trichogramma dendrolimi Matsumura is widely used as a biological control agent of many lepidopteran pests. Diapause has been used as an effective method to preserve the Trichogramma products during mass rearing production. However, it currently takes at least 70 days to break diapause, and we tested whether gradually increasing the temperature instead of using constant temperature could reduce the time of diapause termination and offer a higher flexibility to Trichogramma producers. The diapause termination rates of individuals kept at different conditions were measured, and five groups for which diapause termination rate reached the 95% were selected to test five biological parameters, including the number of eggs parasitized, the parasitism and emergence rates, the female sex ratio, the wing deformation rate, and the parasitoid longevity. Compared to the currently used procedure (70 days at 3 °C), treatments with at least two different temperatures resulted in higher parasitism and emergence rates while keeping the other parameters constant. The treatment that consisted of at least two different temperatures preceded by only 55 days of induction period had the highest population trend index, meaning that the population under these conditions grows more rapidly. Our results demonstrate that gradually increasing temperature allows T. dendrolimi to complete diapause earlier than at present while increasing its potential pest control capacity and providing additional flexibility in mass production of T. dendrolimi.

Conditions to Terminate Reproductive Diapause of a Univoltine Insect: Ceratapion basicorne (Coleoptera: Apionidae), a Biological Control Agent of Yellow Starthistle

Ceratapion basicorne (Illiger) is a recently approved univoltine biological control agent that develops inside the rosette of yellow starthistle (Centaurea solstitialis L.), an invasive annual plant. Adult weevils normally emerge in early summer, and females are thought to be in reproductive diapause until the following spring, when they oviposit in rosettes. The long period of reproductive diapause constrains mass-rearing this weevil because only one generation per year can be produced. Determining the environmental conditions that regulate diapause termination may enable shortening diapause under laboratory conditions to increase production of adults to release. We tested three hibernating conditions (greenhouse [ambient temperature and photoperiod], glass door refrigerator [5°C and ambient photoperiod], and growth chamber [5°C and 24 h dark]) for three durations (4, 8, 11 wk). The highest proportion of females laying eggs came from the growth chamber, with 40% terminating diapause after 4 wk, 80% after 8 wk, and 95% after 11 wk of exposure. Our study demonstrates that duration of cold temperature is an important stimulus to terminate reproductive diapause of C. basicorne, and that exposure to ambient light had no effect at 5°C. However, 47% of females held at ambient greenhouse conditions, without any chilling period, completed diapause within 11 wk. Thus, a cold period can accelerate diapause development, but it is not necessary for its completion. Reducing the winter diapause period from about 6 mo to 11 wk should enable the production of multiple generations per year to increase the number of insects available to release.

Reproductive Dormancy in Overwintering Adult Eucryptorrhynchus brandti (Coleoptera: Curculionidae)

Dormancy is important for overwintering insects to resist and adapt to adverse conditions. Dormancy generally contains quiescence and diapause. Eucryptorrhynchus brandti Harold (Coleoptera: Curculionidae), tree-of-heaven trunk weevil (TTW), is a destructive pest and highly host-specific to Ailanthus altissima in China. TTW has one generation per year and overwinters as both larvae and adults. In this study, to examined dormancy type of adults and find a method to store overwintering adults, we collected adults from 20 October 2018 to 13 March 2019. We studied the behavior and reproductive development of adults under field cold conditions for 0 and 10 d and laboratory warm conditions for 5 and 10 d. We recorded developing eggs in females, and the clarity of the testis edge, the yellow point in the testis lobe, the ratio of the inner content in the accessory gland, and the accessory gland color in males. Adults transferred from the field to the laboratory had resumed reproductive development directly. Results indicated that the dormancy type of TTW adults was quiescence. Adults stored in the field were still in a dormant state and the field-storage method was effective. Current study provided basic data for controlling overwintering TTW adults and solve the storage of insect sources during the winter.

Temperature and Photoperiodic Response of Diapause Induction in Anastatus japonicus, an Egg Parasitoid of Stink Bugs

Simple Summary

Anastatus japonicus Ashmead is an important egg parasitoid wasp and natural enemy of Tessaratoma papillosa. It is commercially used in biological control, and has effectively suppressed T. papillosa population for decades in China. In practice, medium temperature in spring always induces diapause in A. japonicus, leading to the delay of adult emergence and missing the best chance to control the pest. Accurately regulating the development of A. japonicus is a key technique for the production of biocontrol agents and field release applications. In this study, responsible environmental factors for the induction of diapause in A. japonicus were investigated. A short photoperiod and medium temperature led to diapause in A. japonicus. Second–third instar larva are the most sensitive stages to diapause stimuli. Diapausing mature larvae had a significantly higher survival rate after 180 days storage at 10 °C than that of nondiapausing mature larvae. Taken together, results suggest methods that could be exploited in the developmental regulation, field-release pretreatment technology, and long-term storage of A. japonicus.

Abstract

Anastatus japonicus Ashmead is a widely used biological control agent against stink bugs that can be successfully reared using the large eggs of the Chinese silkworm. In this study, environmental factors responsible for the induction of diapause in A. japonicus were investigated on host eggs of the Chinese silkworm. A. japonicus exhibited a facultative, mature larval diapause within its host eggs. Second–third instar larva are the most sensitive stages to diapause stimuli. The accumulation of diapause stimuli during all the larval stages maximized the diapause response. A short photoperiod of 10L:14D and temperature of 17 °C led to the occurrence of the highest diapause response, while a long photoperiod (14L:10D) and low temperatures (11 and 14 °C) prevented the diapause. A specific exposure period was required to reach high diapause incidence. Diapausing mature larvae had a significantly higher survival rate after 180 days storage at 10 °C than that of nondiapausing mature larvae. Taken together, results suggest methods that could be exploited in the developmental regulation, field-release pretreatment technology, and long-term storage of A. japonicus.

[Effect of temperature on post-diapause reproductive development in Listronotus maculicollis (Coleoptera: curculionidae)]

The annual bluegrass weevil Listronotus maculicollis requires chilling exposure to terminate reproductive diapause during overwintering, but the effects of temperature on its post-diapause development in spring remain unclear. To explore this effect, overwintering adults were transferred from cold conditions (6°C/4°C, L:D 10:14) to different warm-up temperatures at L:D 12:12. When weevils were transferred to 7, 14 and 21°C in December and late January, the sizes of male and female reproductive organs were significantly smaller at 7°C than at 14 and 21°C. When weevils were transferred to 7, 9, 11, 13 and 15°C in late January, higher temperatures facilitated the post-diapause development. In both sexes, the sizes of reproductive organs and developmental rate increased with temperature. Reproductive organs did not grow significantly at 7°C in males and at 7-9°C in females, at which the percentage of developing weevils remained low. The time required for 50% of individuals to resume development was 44, 18, 13 and 8 days at 9, 11, 13 and 15°C, respectively, in males and 19, 14 and 8 days at 11, 13 and 15°C, respectively, in females. The threshold temperature for post-diapause development was 7.8°C in males, based on which 61.7 degree-days coincided with 50% of individuals developing. Under field conditions, the percentage of male and female maturity and insemination rate were low until early March, but all reached 100% by late March.

Supplementary Nutrition of Eucryptorrhynchus brandti (Coleoptera: Curculionidae: Cryptorrhychinae): Effect of Ailanthus altissima Host Tissues on Ovary Maturation and Oviposition

Eucryptorrhynchus brandti is a trunk-boring pest that causes severe damage to its single host, Ailanthus altissima in China. Eucryptorrhynchus brandti adults require supplementary nutrition to achieve sexual maturity. Adults feed on petioles, shoots, 2- to 3-yr-old branches, and stems of A. altissima, but prefer to feed on stems. The effects of different host plant tissues on ovary development and oviposition in E. brandti are unclear. Thus, we examined the effects of the consumption of A. altissima petioles, shoots, branches, and stems on ovary development of E. brandti. Using ovaries of different ages, we also evaluated the differentiation of ovarioles, presence and appearance of follicular relics, number of eggs, cuticle hardness, and fat body quantity over time. We found the reproductive system was composed of two ovaries, lateral oviducts, a common oviduct, and spermatheca. The physiological age of ovaries was divided into three nulliparous and three parous stages. Females fed on stems had mature ovaries and successfully oviposited; females fed on other tree tissues or water had no functional ovaries and did not lay eggs. We found virgin females must obtain sufficient nutrition, as evidenced by the immature ovaries in females fed the water. We also found that only stems of A. altissima contribute to egg development; however, the other tree tissues do not contribute to egg development. These findings improve our understanding of the feeding preference of E. brandti from the perspective of ovary development as well as optimal host tissues for ovary development.

Diagnostic Dose Assays for the Detection and Monitoring of Resistance in Adults From Listronotus maculicollis (Coleoptera: Curculionidae) Populations

The annual bluegrass weevil, Listronotus maculicollis (Kirby) (Coleoptera: Curculionidae), is the most difficult to control insect pest on golf courses in eastern North America. Insecticide resistance, particularly to pyrethroids, is a serious and expanding issue in its management. Optimal diagnostic tools for resistance detection are crucial for efficient resistance monitoring and mitigation. Developed vial and Petri dish assays clearly separated different resistance levels among weevil populations. With the pyrethroid bifenthrin, susceptible, moderately resistant (resistance ratios, RR50s 12.2-95.7), and highly resistant (RR50s 258.2-1760.9) populations were distinguished. With the organophosphate chlorpyrifos, susceptible, tolerant (RR50s 2.4-6.7), and resistant (RR50s 8.8-120.7) populations were distinguished. In validation assays, several bifenthrin and chlorpyrifos concentrations were needed to separate resistance levels in Petri dish (bifenthrin: 112.2 and 336.3 or 3,362.5 mg AI/m2; chlorpyrifos: 3.4 and 33.6 mg AI/m2) and vial (bifenthrin: 112.1 or 1,120.8 mg AI/m2; chlorpyrifos: 2.2 and 11.2 mg AI/m2) assays. The Petri dish assay with formulated bifenthrin and chlorpyrifos was the best option for L. maculicollis resistance detection and monitoring. It demonstrated sufficient discriminating power, accurately reflected resistance levels, and was easier to conduct. A single diagnostic concentration sufficed to separate susceptible and resistant populations. To determine different resistance or tolerance levels, two to three concentrations were necessary.

Pyrethroid-Resistance Level Affects Performance of Larvicides and Adulticides From Different Insecticide Classes in Populations of Listronotus maculicollis (Coleoptera: Curculionidae)

The annual bluegrass weevil, Listronotus maculicollis Kirby (Coleoptera: Curculionidae), is a major pest of golf course turf in eastern North America with widespread insecticide resistance. This study examined the effect of pyrethroid-resistance level on the efficacy of adulticides and larvicides from different insecticide classes commonly used for L. maculicollis management through greenhouse and field studies. The tested populations had previously been determined to be susceptible, moderately resistant, resistant, and highly resistant to the pyrethroid bifenthrin. Targeting adults, efficacy was significantly reduced for bifenthrin against the highly resistant population and for the spinosyn spinosad and the oxadizine indoxacarb against the resistant and highly resistant populations. Efficacy of the organophosphate chlorpyrifos was not significantly reduced. No adulticide provided significant control of resistant and highly resistant populations. Targeting larvae, the efficacy of spinosad and the anthranilic diamide cyantraniliprole was marginally reduced against the highly resistant population. Significant reductions in efficacy and no significant control were observed for indoxacarb against the highly resistant population and for the neonicotinoid clothianidin, the anthranilic diamide chlorantraniliprole, and the organophosphate trichlorfon against the resistant and highly resistant populations. Our findings lay the groundwork for management recommendations for populations with different resistance levels. Generally, synthetic insecticide applications should be minimized with greater use of larvicides supplemented with nonchemical control alternatives. The remaining effective larvicides should be rotated using cyantraniliprole, spinosad, and indoxacarb against resistant and cyantraniliprole and spinosad against highly resistant populations.

Role of Plant Volatiles in Host Plant Recognition by Listronotus maculicollis (Coleoptera: Curculionidae)

The annual bluegrass weevil (ABW), Listronotus maculicollis Kirby, is an economically important pest of short cut turfgrass. Annual bluegrass, Poa annua L., is the most preferred and suitable host for ABW oviposition, larval survival and development. We investigated the involvement of grass volatiles in ABW host plant preference under laboratory and field conditions. First, ovipositional and feeding preferences of ABW adults were studied in a sensory deprivation experiment. Clear evidence of involvement of olfaction in host recognition by ABW was demonstrated. Poa annua was preferred for oviposition over three bentgrasses, Agrostis spp., but weevils with blocked antennae did not exhibit significant preferences. ABW behavioral responses to volatiles emitted by Agrostis spp. and P. annua were examined in Y-tube olfactometer assays. Poa annua was attractive to ABW females and preferred to Agrostis spp. cultivars in Y-tube assays. Headspace volatiles emitted by P. annua and four cultivars of Agrostis stolonifera L. and two each of A. capillaris L. and A. canina L. were extracted, identified and compared. No P. annua specific volatiles were found, but Agrostis spp. tended to have larger quantities of terpenoids than P. annua. (Z)-3-hexenyl acetate, phenyl ethyl alcohol and their combination were the most attractive compounds to ABW females in laboratory Y-tube assays. The combination of these compounds as a trap bait in field experiments attracted adults during the spring migration, but was ineffective once the adults were on the short-mown turfgrass. Hence, their usefulness for monitoring weevil populations needs further investigation.